Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of fiber optic networks to support the rapid growth in data and voice traffic applications. A WDM system employs plural optical signal channels, each channel being assigned a particular channel wavelength. In a WDM system, signal channels are generated, multiplexed, and transmitted over a single waveguide, and demultiplexed to individually route each channel wavelength to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance optical systems.
Proposed wavelength division multiplexed optical communication systems typically include multiplexer and demultiplexer switching elements which permit only a fixed number of optical channels to be used in the optical system. In one optical system configuration, for instance, the multiplexed signal is broken down into its constituent optical signals through the use of an integrated frequency router demultiplexer. The frequency router uses silicon optical bench technology in which plural phosphorus-doped silica waveguides are disposed on a silicon substrate. An optical star outputs to an array of N waveguides having adjacent optical path lengths which differ by q wavelengths; this array in turn feeds an output N×N star. Such a frequency router design for an optical communication system is described in Alexander et al., J. Lightwave Tech., Vol. 11, No. 5/6, May/June 1993, p. 714. Using a 1×N configuration at the input, a multiplexed optical signal containing light of different frequencies is separated into its component frequencies at each waveguide extending from the output N×N star. Although this configuration adequately separates light of different frequencies, the integrated optical design fixes both the number and the respective wavelengths of the optical channels. Consequently, adding or decreasing the number of optical channels or changing the channel wavelength or spacing is not possible without providing a completely new demultiplexing switching element to the optical network. In other words, the scalability of such networks is limited because of the switching element's lack of flexibility.
One area where this lack of flexibility manifests itself is in connection with the provisioning of a backup path through the network in the event of equipment failure. For example, in the aforementioned WDM transmission system, since each channel wavelength is assigned its own path through the switching element, it is not possible to reroute a given channel wavelength along a different path should a failure occur in the transmitter that generates the given wavelength. In particular, it is not possible to substitute for the failed transmitter a backup transmitter that resides on another of the switching element's input ports unless the backup transmitter operates on its own channel wavelength that is different from the wavelength at which the failed transmitter operates. As a result, when it becomes necessary to use the backup transmitter a new path must be established through the entire network to accommodate the change in channel wavelength. Unfortunately, the provisioning of a backup path can be a slow process requiring inter-node communication and processing, which not only slows down the restoration process, but which may also disturb other traffic in the system.
Accordingly, it would be desirable to provide an optical communication system in which a backup path can be provisioned through the system in the event of a transmitter or receiver failure that allows restoration to be accomplished in a more rapid and less disruptive manner than in the aforementioned system.